A survey on rate of illness among 10 500 tourists returning from the tropics and a control group of 1300 from the United States and Canada was performed by a questionnaire completed during the flight back to Switzerland. Only 25% from southern destinations, but 53% from North America, were never incapacitated during their travels. The vast majority of the illnesses were of no consequence, as was reflected by a low rate for bed confinement of 4% and 1% respectively. The most frequent symptoms in the tropics were diarrhea in 34% (mostly of short duration and mild degree), constipation in 14%, upper respiratory illness in 12% and insomnia in 11%. There were significant differences according to sex, age and tropical experience. The influence on pre-existing diseases was assessed. 7% of the tourists returning from the tropics were sick in the course of the year following the journey.